In the heart of Singapore, there is a sprawling, vacant plot of private land 30 times the size of The White House. Just down the road from the US embassy and bordering condominiums worth as much as S$4.5 million per unit, the swathe of untended jungle hides the remains of two abandoned palaces and would be worth a cool S$4.7 billion if sold for development, according to one estimate. In a city where...